1. Field of the Invention
The present invention relates to a heat treatment jig that is used in a heat treatment boat of a vertical heat treatment furnace, and in more detail, to a heat treatment jig for silicon semiconductor substrates that can retard the generation of slips as crystal defects during heat treatment at high temperature and can prevent the generation of surface steps attributable to the growth hindrance of the thermally oxidized film which generates at the back surface of said silicon semiconductor substrates when being held in oxidizing atmosphere in high temperature heat treatment.
2. Description of the Related Art
A silicon semiconductor substrate that is processed in LSI device fabrication process, in fabrication steps such as oxidation, diffusion, film deposition and the like, is repeatedly subjected to high temperature heat treatment.
In heat treatment of silicon semiconductor substrates, a vertical heat treatment furnace can reduce an installation space and is suitable for simultaneous heat treatment of a large quantity of semiconductor substrates with large diameter; accordingly, it is adopted as an apparatus to be used in various heat treatments of semiconductor substrates.
FIG. 1 is a diagram showing a configuration example of a heat treatment boat for silicon semiconductor substrates that is used in a vertical heat treatment furnace. A heat treatment boat 1 includes three pillars 3 or more along with top and bottom panels 5, 6 that fix said pillars 3 at top and bottom positions, and is provided with an opening segment 2. The pillars 3 are provided with substrate support means 4, so that, after the silicon semiconductor substrates are charged from the opening segment 2 on the substrate support means 4, the boat is inserted into the vertical heat treatment furnace, being followed by predetermined heat treatment.
As shown in FIG. 1, a heat treatment boat 1 is constituted of a pair of top and bottom panels 5, 6 disposed with a space and a plurality of pillars 3 that link these top and bottom panels; accordingly, an opening segment 2 is indispensably provided to charge the silicon semiconductor substrates on the substrate support means 4 or to discharge those therefrom. Accordingly, two pillars 3 disposed at the opening segment 2, in order to facilitate charging or discharging of silicon semiconductor substrates, are normally put in place with a space equivalent to the diameter of silicon semiconductor substrate.
During high temperature heat treatment at 1100° C. or more in above-mentioned vertical heat treatment furnace for silicon semiconductor substrates, when an in-plane temperature distribution becomes inhomogeneous in silicon semiconductor substrates, the generation of thermal stress is accompanied in silicon semiconductor substrates. Further, depending on a supporting method for silicon semiconductor substrates, owing to its own weight of wafer (substrate), the stress (hereinafter referred to simply as “tare stress”) happens to be generated.
In conventional heat treatment boat, when a silicon semiconductor substrate is merely supported at its periphery portion and the silicon semiconductor substrate with 300 mm diameter is heat treated, the tare stress is markedly generated, resulting in causing difficulty to be used.
It is well known that the thermal stress and the tare stress develop crystal defects, which are called the slips, in the silicon semiconductor substrate during heat treatment. As the slips cause an increase in the leakage current of the LSI device and deteriorate the planarity of silicon semiconductor substrate, there has been proposed by many to suppress the thermal and tare stress in view of quality control of silicon semiconductor substrates up to date.
For instance, in Japanese Patent Application Laid-open Publication No. 10-242067 and in Japanese Patent Application Laid-open Publication No. 2003-100648, a heat treatment jig having a disc structure that supports the entire back surface of silicon semiconductor substrate with a solid flat plate is disclosed. However, in this heat treatment jig having a disc structure, the local temperature difference is likely to be developed at the region where silicon substrate as a work piece to be heat treated makes plane contact with said jig, leading inadvertently to generate the slips attributable to the incurred thermal stress.
To solve above problem in the heat treatment jig having a disc structure, many proposals that adopt the heat treatment jig having a ring structure come to be made, which is represented by Japanese Patent Application Laid-open Publication No. 09-199438. In this representative, it is disclosed that silicon substrates are supported at peripheral portions of the back surface thereof in using a heat treatment jig having a ring-like tray. However, the present inventors made investigations and took note that, in using said heat treatment jig having a ring-like tray, the deflection was developed at the central portion of silicon substrate in accompany of tare stress generation, thereby the silicon substrate came to contact with the end face of inner peripheral portion in ring-like tray, resulting in generation of surface defects thereon which became the origin of slip generation.
Similarly, in Japanese Patent Application Laid-open Publication No. 2001-060559, as a countermeasure to above problem, in order to retard the generation of tare stress in silicon substrate, a heat treatment method is proposed to use the jig, wherein a first ring, which is protruded in ring-like manner, together with a second ring, which is also protruded in ring-like manner and disposed in concentric-circle shape arrangement at the outside of said first ring, support silicon substrates at the back surface thereof. However, the present inventors made investigations and noted that, as the flatness of these dual rings, namely a first ring and a second ring, was not specified, it was not possible to equally support silicon substrates and to prevent the generation of the slips.
In Japanese Patent Application Laid-open Publication No. 2003-197722, a heat treatment jig having a ring-like structure (inclusive of horseshoe-like jig) is disclosed, wherein the flatness and surface roughness are stipulated as to the contact surface, and wherein the configuration is such that a plurality of through-wall holes, which are disposed in concentric-circle shape, are provided on the support face thereof for silicon substrates, while the total section area of through-wall holes on the support face for silicon substrates is stipulated. Also in this publication, it teaches that the surface roughness Ra is preferred to be 0.1 to 0.7 μm, and so the flatness to be 50 μm or less.
However, when it comes to the heat treatment jig to be applied for silicon semiconductor substrates of 300 mm diameter, high accuracy machining is required to secure the flatness to be 50 μm or less over the entire region where the silicon substrates to sit on, which results in the cost increase and can not be practically manufacturing parameters with respect to the machining/processing accuracy.